Expose future response in new AsyncServerResponse

This commit introduces AsyncServerResponse, an extension of
ServerResponse that is returned from ServerResponse.async and that
allows users to get the future response by calling the block method.
This is particularly useful for testing purposes.

spring-webmvc/src/test/java/org/springframework/web/servlet/function/DefaultAsyncServerResponseTests.java

@@ -0,0 +1,39 @@
+/*
+ * Copyright 2002-2020 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.web.servlet.function;
+
+import java.util.concurrent.CompletableFuture;
+
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * @author Arjen Poutsma
+ */
+class DefaultAsyncServerResponseTests {
+
+	@Test
+	void block() {
+		ServerResponse wrappee = ServerResponse.ok().build();
+		CompletableFuture<ServerResponse> future =  CompletableFuture.completedFuture(wrappee);
+		AsyncServerResponse response = AsyncServerResponse.create(future);
+
+		assertThat(response.block()).isSameAs(wrappee);
+	}
+
+}